1. Field of the Invention
The present invention relates to sound producing microphones and more particularly to a condenser microphone formed with two different sound producing diaphragms on opposite sides of a back plate, which are operable independently to produce different sound reproduction characteristics.
2. Description of the Prior Art
It is often desired that an audio system produce a wide variety of sounds depending upon the particular material being played, the location of the sound system and/or the mood desired by the listeners. Most often, these types of changes in audio output are generated and regulated by adjustments to the amplifier audio settings of the base and treble circuits, in addition to adjusting the volume control.
In the recording process, the diaphragms of the microphones generate the original sound. Diaphragms of different materials, conductive coatings, thicknesses or stiffnesses have a marked effect on the character of the sound that is ultimately heard by the listener. Recording engineers select a particular microphone to enhance, or make more pleasing to the listener the voice or instrument being recorded.
In a condenser microphone, the capsule includes a diaphragm spaced from a back plate. The diaphragm acts as one plate of a capacitor and the diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically, when the plates are closer together, capacitance increases and a charge current occurs. When the plates are further apart, capacitance decreases and a discharge current occurs. A voltage is required across the capacitor for this to work. This voltage is supplied either by a battery in the microphone or by external phantom power source from the equipment to which the microphone is connected.
Since the plates are biased with a fixed charge (), the voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation: Q=C×V where Q=charge in coulombs, C=capacitance in farads and V=potential difference in volts. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor.
Condenser microphones produce high quality audio signals and are a popular choice in laboratory and studio recordings and range in cost from inexpensive to very expensive. Condenser microphones are available with two capsules wherein the signals from each are electrically connected to provide a range of polar patterns. Polar patterns are a graphical representation of the microphone's directionality.
Every microphone has a property known as directionality. This describes the microphone's sensitivity to sound from various directions. Some microphones pick up sound equally from all directions, while others pick up sound only from one direction or a particular combination of directions. The types of directionality are divided into three main categories:                1) Omnidirectional, which picks up sound evenly from all directions.        2) Unidirectional, which picks up sound predominantly from one direction and includes cardioid, i.e., heart shaped patterns.        3) Bidirectional, which picks up sound from two opposite directions.        
In other words, the term polar pattern is used to describe the response of a microphone to sound sources from various directions. Each type of polar pattern has its own place and usage in the recording process. Generally, microphones tend to become more directional in focus as frequencies increase. In other words, diaphragms are generally less sensitive to high frequencies off axis. The cardioid is the most common polar pattern found in microphones. Cardioids pick up sound primarily from the front of the diaphragm. The back of the diaphragm rejects sound, allowing the engineer to isolate the signal source from other performance elements or background noise. Omni is used to capture room ambience and reflections along with the source, thereby yielding a more open sound compared to the more focused quality of cardioid. Omni is desirable for vocal groups, Foley sound effects, and realistic acoustic instruments. Omni also exhibits significantly less proximity effect than cardioids.
In multi-pattern microphones, plural diaphragms may be used, of the same or different materials, however, all diaphragms are polarized and operate at the same time to create the multi-pattern.
With the foregoing in mind, a need exists for an improved condenser microphone providing the user with the added versatility of two different sounding cardiod pattern microphones in one. The present invention provides such a microphone.